1. Field of the Invention
This invention relates generally to placement of cutting elements on a rotary drilling tool for use in drilling subterranean formations or other hard materials disposed within a subterranean formation, such as drill strings, casing components, and the like. More particularly, the invention pertains to placement of two or more redundant cutting elements upon a drilling tool so as to contact a change in formation characteristics between different subterranean regions between a formation and another structure disposed therein, or between two structures disposed in a borehole prior to contact by other cutting elements disposed thereon.
2. Background of Related Art
Conventionally, it is well-known that cutting elements located in the different positions on a face of a rotary drill bit may experience vastly different loading conditions, different wear characteristics, or both. The effects of the loading and wear have been accommodated in conventional rotary drill bits by variations in cutting element size, geometry, and configuration in relation thereto. However, conventional approaches to cutting element placement on a rotary drill bit often do not consider the effects and conditions of the cutting elements as well as the forces and torques associated therewith during an initial encounter of a transition during drilling between two adjacent subterranean formations having at least one differing characteristic. In addition, conventional approaches for cutting element placement on a rotary drill bit have not adequately addressed considerations of transitions occurring when drilling through downhole equipment, such as a casing shoe, the cement surrounding the casing shoe, and the formation therebelow.
Several approaches have been developed to accommodate varying loading conditions that may occur in different positions on a rotary drill bit face. For instance, U.S. Pat. Nos. 6,021,859, 5,950,747, 5,787,022, and 5,605,198 to Tibbitts, and Tibbitts et al., respectively, each of which is assigned to the assignee of the present invention, disclose selective placement of cutting elements of differing diamond table-to-substrate interface design at different locations on the bit face, to address different predicted or expected loading conditions.
In a conventional approach to improving the drilling performance of rotary drill bits, U.S. Pat. Nos. 6,164,394 and 6,564,886 to Mensa-Wilmot et al. each disclose rotary drill bits including cutting elements disposed at substantially identical radial positions wherein the rotationally preceding cutting element is oriented at a positive backrake angle, while the rotationally following cutting element is oriented at a negative backrake angle and exhibits less exposure than the rotationally preceding cutting element.
Similarly, U.S. Pat. No. 5,549,171 to Mensa-Wilmot et al. discloses a rotary drill bit, including sets of cutting elements mounted thereon, wherein each set of cutting elements includes at least two cutting elements mounted on different blades at generally the same radial position but having differing degrees of backrake and exposure.
Further, U.S. Pat. No. 4,429,755 to Williamson discloses a rotary drill bit including successive sets of cutting elements, the cutting elements of each set being disposed at equal radius from and displaced about the axis of rotation of the rotary drill bit through equal arcs, so that each cutting element of a set thereof is intended to trace a path which overlaps with the paths of adjacent cutting elements of other set or sets of cutting elements.
Also, U.S. Patent Application 2002/0157869 A1 to Glass et al. discloses a fixed-cutter drill bit, which is purportedly optimized so that cutter torques are evenly distributed during drilling of homogeneous rock and also in transitional formations. Methods utilizing predictive mathematical drilling force models are also disclosed.
Rotary drill bits, and more specifically fixed cutter or “drag” bits, have also been conventionally designed as so-called “anti-whirl” bits. Such bits use an intentionally unbalanced and oriented lateral or radial force vector, usually generated by the bit's cutters, to cause one side of the bit configured as an enlarged, cutter-devoid bearing area comprising one or more gage pads to ride continuously against the side wall of the well bore to prevent the inception of bit “whirl,” a well-recognized phenomenon wherein the bit precesses around the well bore and against the side wall in a direction counter to the direction in which the bit is being rotated. Whirl may result in a borehole of enlarged (over gauge) dimension and out-of-round shape and in damage to the cutters and bit itself.
U.S. Pat. Nos. 5,010,789 and 5,042,596 to Brett et al., the disclosures of each of which are incorporated in their entirety by reference thereto, disclose anti-whirl drill bits. Further, U.S. Pat. No. 5,873,422 to Hansen et al., assigned to the assignee of the present invention and the disclosure of which is incorporated in its entirety by reference thereto, discloses support structures in a normally cutter devoid zone to stabilize the drill bit.
In a further approach to stabilize rotary drill bits while drilling, selective placement of cutting elements upon a rotary drill bit may create stabilizing grooves, kerfs, or ridges. Such configurations are intended to mechanically inhibit lateral vibration, assuming sufficient vertical or weight-on-bit force is applied to the rotary drill bit.
For instance, U.S. Pat. No. 4,932,484 to Warren et al. discloses forming a groove by placing a cutting element offset from the other cutting elements positioned along a cutting element profile. Also, U.S. Pat. No. 5,607,024 to Keith et al. discloses cutting elements having differing regions of abrasion resistance. Such a configuration is purported to laterally stabilize the rotary drill bit within the borehole because as the cutting elements wear away, radially alternating grooves and ridges may be formed.
However, despite the aforementioned conventional approaches to improving drilling performance of a rotary drill bit or other drilling tool by configuring the placement or design of the cutting elements thereon, there remains a need for improved apparatus and methods for drilling with a rotary drill bit between differing materials or formation regions with different properties.